1. Field of the Invention
The present invention relates to a coupled FBAR filter usable for a radio frequency circuit of a wireless device or the like, and more particularly to an FBAR filter using a plurality of modes, and a duplexer and a communication device including the same.
2. Description of the Background Art
Conventionally, filters mounted on wireless communication devices such as cellar phones and the like include dielectric filters, laminated filters and acoustic filters. Known acoustic filters include a monolithic crystal filter (MCF) using a plurality of modes of a bulk wave and a surface acoustic wave filter (SAW filter). Recently, filters are required to have a smaller size, to provide a higher performance and to be usable at a higher frequency. As a device for fulfilling these requirements, a film bulk acoustic resonator filter (FBAR filter) using a bulk wave of a piezoelectric thin film has been developed.
A conventional MCF using a plurality of modes (Japanese Laid-Open Patent Publication No. 10-163804, FIG. 1) will be described. This type of MCF typically uses an AT cut crystal substrate. FIG. 26 shows a structure of the conventional MCF.
The MCF has the following structure. An AT cut crystal substrate 91 is photo-etched to integrally form a vibration portion 92 of a super thin plate and a thicker ring portion 93 for supporting the vibration portion 92. On a flat surface of the AT cut crystal substrate 91, electrodes 94a and 94b are provided with a gap g interposed therebetween. On the entirety of a surface of the AT cut crystal 91 having a recessed portion, an electrode 95 is applied by vapor deposition or the like. Owing to such a structure, a first-order mode and a second-order mode which are generated as a result of acoustic coupling between two electrode pairs, i.e., the electrode pair 94a/95 and the electrode pair 94b/95 can be used. The coupling degree of the two modes, i.e., the resonant frequency difference between the two modes depends on the elastic constant of the AT cut crystal substrate 91, the shapes of the electrodes, thicknesses of the electrodes, and the size of the gap g between the two electrode pairs. As the coupling degree is higher, a filter which has a smaller loss and is usable over a wider band can be realized.
Another MCF using a plurality of modes (Japanese Laid-Open Patent Publication No. 10-145181, FIG. 1) will be described. FIG. 27 shows a structure of the another conventional MCF.
The MCF includes three pairs of electrodes, i.e., an electrode pair 82/83, an electrode pair 84/85 and an electrode pair 86/87, each including the two electrodes facing each other with a crystal substrate 81 (piezoelectric substrate) interposed therebetween. The central electrode pair 84/85 is an input or an output. The two electrode pairs 82/83 and 86/87 located on the sides of the electrode pair 84/85 and having generally the same shape with each other are connected in parallel and act as an output or an input. The MCF excites a first-order mode thickness vibration and a third-order mode thickness vibration and thus realizes a high coupling degree of the plurality of modes. Thus, a filter usable over a wide band is realized.
A filter using an FBAR, which uses a piezoelectric thin film to be usable at a higher frequency, has been proposed. FIG. 28 shows a cross-sectional view of a conventional coupled FBAR filter using an FBAR and ideal vibration mode distributions thereof. The FBAR includes a lower electrode, a piezoelectric thin film, an upper electrode and the like stacked on a substrate by means of sputtering or the like, and thus has a different basic structure from that of an MCF.
As shown in FIG. 28, a coupled FBAR filter 70 includes two electrode pairs, i.e., an electrode pair 74/75 and an electrode pair 76/77, which are provided on a substrate 71 while interposing a piezoelectric thin film 73 therebetween. Owing to such a structure, two input/output vibration portions 78 and 79 are provided. The substrate 71 has a cavity 72 formed therein which is covered with the two input/output vibration portions 78 and 79. The cavity 72 is provided for guaranteeing vibrations of the two vibration portions 78 and 79. As means for guaranteeing the vibrations, an acoustic mirror layer may be used. Owing to this, the two input/output vibration portions 78 and 79 share the same cavity 72 or acoustic mirror layer, and function as a filter using the coupling of the first-order mode and the second-order mode. It is known that the coupling degree of the two modes is improved by reducing the sizes of the electrodes, the thicknesses of the electrodes, and the distance between the two input/output vibrations (two electrode pairs), so that the filter obtains smaller-loss and wider-band characteristics.
FIG. 29 shows a cross-sectional view of a conventional coupled FBAR filter for coupling a first-order mode and a third-order mode and ideal vibration mode distributions thereof. A coupled FBAR filter 60 shown in FIG. 29 includes three input/output vibration portions 61 through 63. Owing to this, a filter using the coupling of the first-order mode and the third-order mode is realized. It is known that the coupling degree of the two modes is improved by reducing the sizes of the electrodes, the thicknesses of the electrodes, and the distance between the three input/output vibrations (two electrode pairs), so that the filter obtains smaller-loss and wider-band characteristics.
An integral value of the vibration mode distributions curves shown in each of FIG. 28 and FIG. 29 is generally equivalent to the amount of charge generated in the piezoelectric thin film. By using the charge generated in the piezoelectric film for the vibration in the two or three vibration portions, the coupling coefficient of each vibration portion is increased and thus a coupled FBAR filter having smaller-loss and wider-band characteristics can be realized.
However, as shown in FIG. 28 and FIG. 29, the distance between the two or three input/output vibration portions (two or three electrode pairs) cannot reduced to less than a certain distance because the distance is restricted by the conditions for producing the electrodes. As a result, a part of the generated charge is consumed by a non-electrode portion as a loss, which deteriorates the filter characteristics.